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Twilight-Zone and Canopy Shade Induction of the Athb-2 Homeobox

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Twilight-Zone and Canopy Shade Induction of the Athb-2 Homeobox Proc. Natl. Acad. Sci. USA Vol. 93, pp. 3530-3535, April 1996 Developmental Biology Twilight-zone and canopy shade induction of the Athb-2 homeobox gene in green plants MONICA CARABELLI*, GIORGIO MORELLIt, GARRY WHITELAMt, AND IDA RUBERTI*§ *Centro di studio per gli Acidi Nucleici, c/o Dipartimento di Genetica e Biologia Molecolare, Universita di Roma La Sapienza P.le Aldo Moro 5, 00185 Rome, Italy; tUnita di Nutrizione Sperimentale, Istituto Nazionale della Nutrizione, Via Ardeatina 546, 00178 Rome, Italy; and TDepartment of Botany, University of Leicester, Leicester LEI 7RH, United Kingdom Communicated by Walter J. Gehring, University of Basel, Basel, Switzerland, December 28, 1995 (received for review October 20, 1995) ABSTRACT We present evidence that a novel phyto- tomorphogenesis) to photomorphogenesis (6-14). By contrast, chrome (other than phytochromes A and B, PHYA and PHYB) in mature green plants, only the phyB mutants show a pheno- operative in green plants regulates the "twilight-inducible" type manifested in changes in shade avoidance, neighbor expression of a plant homeobox gene (Athb-2). Light regula- detection, and flowering (7, 15-17). Thus, PHYB has been tion of theAthb-2 gene is unique in that it is not induced by red proposed to be the major sensor of low R/FR light contrib- (R)-rich daylight or by the light-dark transition but is instead uting to these above developmental processes. However, there induced by changes in the ratio of R to far-red (FR) light. is evidence that PHYB is not solely responsible for these These changes, which normally occur at dawn and dusk responses (16, 18, 19), but as there are no mutants in the other (end-of-day FR), also occur during the daytime under the phytochromes (PHYC, D, and E), it is impossible to test their canopy (shade avoidance). By using pure light sources and respective roles in these and other aspects of plant develop- phyA/phyB null mutants, we demonstrated that the induction ment. of Athb-2 by changes in the R/FR ratio is mediated for the In the present report, we show that changes in light quality most part by a novel phytochrome operative in green plants. (R/FR ratio), which naturally occur at dawn and dusk (end- Furthermore, PHYB plays a negative role in repressing the of-day FR, EOD-FR) and during the daytime under the accumulation ofAthb-2 mRNA in the dark and a minor role in canopy (shade avoidance), specifically induce the expression of the FR response. The strict correlation of Athb-2 expression a plant homeobox gene, Athb-2 (or HAT4) (20, 21). Moreover, with FR-induced growth phenomena suggests a role for the we demonstrated that the induction ofAthb-2 by changes in the Athb-2 gene in mediating cell elongation. This interpretation R/FR ratio is mediated for the most part by a novel phyto- is supported by the finding that the Athb-2 gene is expressed chrome (other than PHYA and PHYB) operative in green at high levels in rapidly elongating etiolated seedlings. Fur- plants. thermore, as either R or FR light inhibits cell elongation in The unique ability of this plant homeobox gene to respond etiolated tissues, they also down-regulate the expression of to changes in light quality suggests a mechanism by which Athb-2 mRNA. Thus, these data support the notion that plants may adapt their developmental programs in response to changes in light quality perceived by a novel phytochrome changes in the light environment. regulate plant development through the action of the Athb-2 homeobox gene. MATERIALS AND METHODS The sessile nature of plants necessitates the ability to sense and Plant Material and Growth Conditions. The Landsberg respond to changes in environmental stimuli such as light. In erecta ecotype of Arabidopsis thaliana L. was the wild type fact, changes in light intensity, quality, or duration affect used in this study; the phyA (phyA-1, ref. 12), phyB (hy3-Bo64, important developmental programs throughout the life cycle ref. 6), andphyA phyB (phyA-2 hy3-Bo64, ref. 13) phytochrome of the plant. The initial event of light perception is carried out mutants were used. Plants were grown in a 16-hr light/8-hr by at least three families of photoreceptors that detect differ- dark cycle for 14 days under a high R/FR ratio (R/FR = 20) ent wavelengths within the visible spectrum: phytochrome as described (22). For experiments with etiolated seedlings, [which detects red (R) and far-red (FR) light], a blue light seeds were plated on MS agar plates and stored in darkness for photoreceptor, and an ultraviolet light photoreceptor. Light 2 days at 4°C; germination was induced by placing the plated signal perception by these receptors activates signaling path- seeds in white light for 2 hr at 21°C and growth proceeded in ways leading to the changes in gene expression that underlie darkness at 21°C for 4.5 days. the physiological and developmental responses important Light Sources. The low R/FR (R/FR = 0.93) ratio treat- throughout the life history of a plant (for review, see ref. 1). ment was performed as described (22). For light-pulse exper- To date, the best characterized photoreceptors are the iments, light sources were from Phylips; the bulb type, filters, phytochromes. The photosensory function of these molecules and fluence rates were as follows: FR, bulb Phylinea (60 W), is based on their capacity for reversible interconversion be- filters, FRF 700 (Rohm & Haas) and Roscolux n. 83 (Rosco), tween R- and FR-light-absorbing forms (for review, see refs. and fluence, 350 /W/cm2; R, bulb TL (40W/15), and fluence 1 and 2). InArabidopsis, five genes encoding the phytochrome 316 IXW/cm2. Measurements of fluence rate in the spectral polypeptides, designated PHYA, B, C, D, and E have been regions were performed by using the Photometer IL 150 identified (3, 4). Studies with photomorphogenic mutants (International Light, Newburyport, MA). deficient in functional PHYA and/or PHYB have shed light on RNA Analysis. RNA was isolated and analyzed as described the functions of these two phytochrome species (for review, see (22). To normalize for RNA loading, filters were stripped and ref. 5). In etiolatedArabidopsis plants, both PHYA and PHYB rehybridized with a 3-ATPase (22) probe. Relative amounts of contribute to the switch from dark-growth development (sko- mRNAs were determined by using an Imaging densitometer, The publication costs of this article were defrayed in part by page charge Abbreviations: FR, far-red; R, red; EOD-FR, end-of-day FR; PHYA, payment. This article must therefore be hereby marked "advertisement" in phytochrome A; PHYB, phytochrome B. accordance with 18 U.S.C. §1734 solely to indicate this fact. §To whom reprint requests should be addressed. 3530 Downloaded by guest on September 26, 2021 Developmental Biology: Carabelli et al. Proc. Natl. Acad. Sci. USA 93 (1996) 3531 GS-670 (Bio-Rad), and relative transcript levels reported are an average of two experiments. RESULTS The FR-Light Induction of Athb-2 Gene Expression Is Mediated by a Phytochrome Other than PHYA or PHYB. We A A.hmi have shown (22) that Athb-2 gene expression was induced in LI mature green plants by exposure to 1 hr of FR-rich light. In the .I present report, we investigated whether any phytochrome is ~wil(l involved in this response. Arabidopsis plants were grown for 2 ty,pe weeks in a normal day/night cycle and the steady-state levels I) of Athb-2 mRNA were monitored after different treatments with pure light sources. A 2-min pulse of pure FR light resulted in a 20-fold increase of the Athb-2 mRNA (Fig. 1A, lane 1) compared to controls (Fig. IA, lane 5). A subsequent pulse of R light reversed this effect (Fig. 1A, lane 3), indicating that this response was indeed attributable to the phytochrome system. B By contrast, a pulse of R light had no effect on the Athb-2 .V ^ " I\ transcript levels (Fig. 1A, lane 2). A subsequent pulse of FR i'l ,1 light showed an enrichment of the Athb-2 mRNA similar to that observed with a pulse of FR light alone (Fig. 1A, lane 4). pli.y We next performed parallel experiments in mutant plants lacking functional PHYA and/or PHYB. Surprisingly, inphyA, l) phyB, or phyA phyB mutant plants, a pulse of FR light was able to induce Athb-2 transcript levels to wild-type levels (Fig. 1 B-D, lanes 1) compared to uninduced controls (Fig. 1 B-D, lanes 5). Furthermore, in all phy mutants, Athb-2 FR-light induction was abolished when the FR-light pulse was followed by a R-light pulse (Fig. I B-D, lanes 3). This FR induction and C Ak.-Ak. R reversibility ofAthb-2 induction observed inphyA, phyB, and *f. *. i'i ".: iA&-.ALI l phyA phyB mutants clearly indicates that a novel phytochrome other than PHYA or PHYB mediates the Athb-2 response to ph.yl FR light. An EOD-FR-Light Treatment Dramatically Affecting De- 1) velopmental Processes Strongly Induces Athb-2 Gene Expres- sion. Time of flowering is developmentally regulated by the length of the day in seed plants. In Arabidopsis, as in other long-day plants, the seedlings show increased hypocotyl elon- gation and an acceleration of flowering in response to FR-light enrichment (e.g., low R/FR ratio). As we showed above that D Athb-2 is artificially induced by a pulse of FR light, we next set 'I out to test whether this gene is induced by an EOD-FR treatment, corresponding to "twilight".
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